1. Field of the Invention
The present invention relates to a constant voltage generator for outputting constant voltage, and more particularly to a constant voltage generator comprising an improved starting circuit, and relates to electronic equipment using such a constant voltage generator.
2. Description of the Related Art
A constant voltage generator is widely used for electronic circuits for securing the accuracy of an analog circuit or decreasing the power consumption of a circuit. One type of constant voltage generator is one using a band gap reference circuit (e.g. Japanese Patent Application Laid-Open No. H3-164916, Japanese Patent Application Laid-Open No. H7-230332). A band gap reference circuit is constructed by combining matched transistors on a semiconductor integrated circuit, and the advantage is that it does not depend on temperature.
A constant voltage generator using a band gap reference circuit requires a transistor for supplying current to the load connected to the output. An example of the circuit format of this transistor is an emitter follower type, where an emitter is connected to the output of the constant voltage generator, however a higher power supply voltage for the amount of forward bias voltage (Vf) between the emitter and the base is required, so this is not appropriate for decreasing the power supply voltage, which is a problem to be described later. Therefore in this description of the related art, a circuit type where a transistor, of which collector is connected to the output of the constant voltage generator, supplies current, will be described. If MOS transistors instead of bipolar transistors constitute the constant voltage generator, a P-type MOS transistor, of which drain is connected to the output of the constant voltage generator, is used to supply current.
FIG. 6 is a circuit diagram depicting a constant voltage generator of the first prior art described in Japanese Patent Application Laid-Open No. H3-164916.
The constant voltage generator 110 of the first prior art is comprised of a band gap reference circuit 111, a current supply circuit 112, a starting circuit 113, a voltage-current conversion circuit 114 and a starting detection circuit 115.
The band gap reference circuit 111 generates the constant voltage (Vref) for the constant voltage generator 110 to output from the output terminal (VREF). The current supply circuit 112 supplies current to the load connected to the output terminal (VREF), and to the above-mentioned band gap reference circuit 111. The starting circuit 113 starts up the band gap reference circuit 111 by forcibly flowing current to the current supply circuit 112 when the power supply voltage (VCC) is started. The voltage-current conversion circuit 114 converts the voltage of the output terminal (VREF) into current, and outputs the current to the current supply circuit 112. And the starting detection circuit 115 detects that the power supply voltage (VCC) started up to prevent the starting circuit 113 from influencing the constant voltage generator 110, which will be described later.
The band gap reference circuit 111 is comprised of resistors 124 and 125 which are connected to the output terminal (VREF) in parallel and have a same resistance value, a diode-connected transistor 121 which is connected to the other end of the resistor 124, a transistor 122 which has a larger emitter-base area (larger current capability) than the transistor 121, and is connected to the other end of the resistor 125 with sharing the base voltage with the transistor 121, a resistor 120 which is connected to the emitter of the transistor 122, and a transistor 123 of which base is connected to the connection point between the resistor 125 and the transistor 122, and of which emitter is grounded. By this configuration, voltage for outputting the constant voltage (Vref) from the output terminal (VREF) is generated.
The current supply circuit 112 is comprised of a resistor 128 and transistor 126, and a resistor 129 and transistor 127, which become a current mirror. These transistors 126 and 127 are PNP types. The transistor 126 supplies current to the output terminal (VREF), and this current is controlled by adjusting the current that flows through the transistor 127.
The starting circuit 113 is comprised of a resistor 130 which is connected to the power supply voltage (VCC), two stages of diodes 131 and 132 which are connected to the resistor 130, a transistor 133 of which base is connected to the connection point between the resistor 130 and diode 131, and a resistor 134 which is connected to the emitter of the transistor 133.
In this starting circuit 113, when the power supply voltage (VCC) starts up, the base voltage of the transistor 133 becomes double the forward bias voltage (Vf) by the two stages of diodes 131 and 132, and the transistor 133 turns ON. In this transistor 133, current, which is determined by the resistance value of the resistor 134, flows, and the current flows to the transistor 127 of the above-mentioned current supply circuit 112. As a result, the current is supplied from the transistor 126 to the output terminal (VREF) and the above-mentioned band gap reference circuit 111, and the band gap reference circuit 111 is started up.
In the starting detection circuit 115, the base voltage of the transistor 133 of the starting circuit 113 is decreased to turn the transistor 133 OFF by the ON current of the transistor 143 after the power supply voltage (VCC) is started up.
The voltage-current conversion circuit 114 is comprised of a transistor 139 of which base is connected to the output terminal (VREF), and a resistor 140 which is connected to the emitter of the transistor 139. The voltage of the emitter of the transistor 139 is lower than the constant voltage (Vref) of the output terminal (VREF) for the amount of the forward bias voltage (Vf), and this voltage is applied to the resistor 140. Therefore after the power supply voltage is started up, the above-mentioned current supply circuit 112 is controlled by current determined by the resistance value of this resistor 140.
In this constant voltage generator of the first prior art, current according to the constant voltage (Vref) of the output terminal (VREF) can be supplied from the current supply circuit 112 to the output terminal (VREF) by using the above-mentioned configuration for the voltage-current conversion circuit 114.
FIG. 7 is a circuit diagram depicting a constant voltage generator of the second prior art described in Japanese Patent Application Laid-Open No. H7-230332. The constant voltage generator 150 of the second prior art is comprised of a band gap reference circuit 151, a current supply circuit 152 and a starting circuit 153. This band gap reference circuit 151 substantially has the same configuration of the band gap reference circuit 111 of the first prior art.
The current supply circuit 152 substantially plays the same function as the current supply circuit 112 of the first prior art, and is comprised of transistors 166 and 167, which become a current mirror. These transistors 166 and 167 are also PNP types. The transistor 166 supplies current to the output terminal (VREF), and the current is controlled by adjusting the current that flows through the transistor 167 using the transistor 163 of the band gap reference circuit 151.
The starting circuit 153 is comprised of a resistor 170 which is connected to the power supply voltage (VCC), two stages of diodes 171 and 172 which are connected to the resistor 170, and a diode 173 which is connected to the connection point between the resistor 170 and the diode 171. This starting circuit 153 substantially plays the same function as the starting circuit 113 of the first prior art, but starting is executed by supplying current directly from the resistor 170 to the band gap reference circuit 151, without using transistors.
The diode 173 of the starting circuit 153 is for preventing the starting circuit 153 from influencing the constant voltage generator 150 after the power supply voltage (VCC) is started up. The output of the transistor 163 of the band gap reference circuit 151 is directly input to the current supply circuit 152.
Therefore in the constant voltage generator 150 of the second prior art, the voltage-current conversion circuit 114 and the starting detection circuit 115 of the first prior art can be omitted, which can make the configuration simpler.